Human Metabolism. Keith N. Frayn

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Название Human Metabolism
Автор произведения Keith N. Frayn
Жанр Медицина
Серия
Издательство Медицина
Год выпуска 0
isbn 9781119331469



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into potential energy of the object. But even maintaining life involves work: the work of breathing, of pumping blood around the vascular system, of chewing food and digesting it. At a cellular level, there is constant work performed in the pumping of ions across membranes, and the synthesis and breakdown of the chemical constituents of cells.

      What we are now discussing is, indeed, metabolic regulation. Metabolic regulation in human terms covers the means by which we take in nutrients in discrete meals, and deliver energy as required, varying from moment to moment and from tissue to tissue, in a pattern which may have no relationship at all to the pattern of intake. Metabolic regulation works ultimately at a molecular level, mainly by modulation of the activities of enzymes. But one should not lose sight of the fact that these molecular mechanisms are there to enable us to lead normal lives despite fluctuations in our intake and our expenditure of energy. In this book, the emphasis will be on the systems within the human body which sense the balance of energy coming in and energy required, particularly the endocrine (hormonal) and the nervous systems, and which regulate the distribution and storage of nutrients after meals, and their release from stores and delivery to individual tissues as required.

      The intention of this preamble is to illustrate that, underlying our everyday lives, there are precise and beautifully coordinated regulatory systems controlling the flow of energy within our bodies. Metabolic regulation is not a dry, academic subject thought up just to make biochemistry examinations difficult; it is at the centre of human life and affects each one of us every moment of our daily lives.

      1.2 The chemistry of food – and of bodies

      Energy is taken into the body in the form of food. The components of food may be classified as macronutrients and micronutrients. Macronutrients are those components present in a typical serving in amounts of grams rather than milligrams or less. They are the well-known carbohydrate, fat, and protein. Water is another important component of many foods, although it is not usually considered a nutrient. Micronutrients are vitamins, minerals, and nucleic acids: they are not oxidised to provide energy, but rather they are used to facilitate biochemical mechanisms of the body. Although these micronutrients play vital roles in the metabolism of the macronutrients, they will not be discussed in any detail in this book, which is concerned with the broader aspects of what is often called energy metabolism.

      The links between nutrition and energy metabolism are very close. We eat carbohydrates, fats, and proteins. Within the body these relatively large molecules are broken down to smaller components, rearranged, stored, released from stores, and further metabolised, but essentially whether we are discussing food or metabolism the same categories of carbohydrate, fat, and protein can be distinguished. This is not surprising since our food itself is of organic origin, whether plant or animal.

      In order to understand metabolism and metabolic regulation, it is useful to have a clear idea of some of the major chemical properties of these components. This is not intended as a treatise in physical or organic chemistry but as a starting point for understanding some of the underlying principles of metabolism. The discussion assumes a basic understanding of the meaning of atoms and molecules, of chemical reactions and catalysis, and some understanding of chemical bonds (particularly the distinction between ionic and covalent bonding).

      1.2.1 Some important chemical concepts

      1.2.1.1 Polarity